1. Field of the Invention
The invention pertains to an apparatus and a method for measuring the insulation resistance of a fuel cell system with respect to a point at electrical ground potential.
2. Description of the Related Art
U.S. Pat. No. 5,760,488 describes a so-called IT architecture (IT: isolated terra) for the load circuit of a vehicle-mounted fuel cell system, in which the load-current lines of the fuel cell system are isolated with respect to the vehicle chassis (vehicle ground). The electrical loads that are supplied by the load-current lines possess a low-resistance connection between their housings and vehicle ground in order to prevent dangerous contact potentials. Thus, the fuel cell voltage provided by the fuel cell system does not refer to vehicle ground potential; one might say that it floats with respect to the ground potential. One of the effects of a certain electrical conductivity of the cooling agent, which is used to cool the fuel cell system, is that the insulation resistance of the load circuit with respect to the vehicle ground is not infinite, but finite.
The present invention starts from a “floating” architecture of the fuel cell system as disclosed in U.S. Pat. No. 5,760,488, but without the restriction to vehicle applications. The invention can in principle be used for any desired stationary or mobile application, although vehicle applications are preferred.
For safety reasons, numerous regulations prescribe a specific lower limit (e.g., 100 kΩ), below which the insulation resistance between the fuel cell system and the ground point must not fall. U.S. Pat. No. 5,760,488 describes the detection of fault situations by means of a bridge circuit placed between the load-current lines and the vehicle ground for the purpose of monitoring the insulation resistance. Normally, with a proper insulation resistance, the bridge circuit does not emit a signal. Only if the insulation resistance drops, does the bridge circuit output a signal, which is then amplified by a differential amplifier and initiates an appropriate response, for example a shutdown of the fuel cell system.
This solution according to U.S. Pat. No. 5,760,488 is not able to determine an absolute magnitude of the insulation resistance, rather it only allows differentiating between cases that are still considered proper and cases that are no longer considered proper.